I ran 466 rwhp on nothing more than erics x-race tune. Stock intake, stock exhaust with the cat and muffler IN place. Dyno'd 270rwhp on stock tuning. Had another 04 6.0 dyno that same day that did 271 on stock tuning and a open exhaust.
Obviously this seems ridiculous, but it is the only dyno run I have. I ran a 1/4 mile run with a dashmount GPS device, with a passenger and a toolbox full of tools, on 34" super swampers (really heavy) and running only on x-street from Eric it calculated 432rwhp or there abouts.
I'd really like to get to an actual track and see what it has. But like Ellerton said, you can make a dyno read whatever you want.